Who knew that falling Treasury yields would be part of the Fed’s tapering?

So much for the higher U.S. Treasury yields financial markets were expecting as the Federal Reserve tapers its monthly bond purchases.

To the contrary, a slew of bullish factors, from a global flight-to-safety trade related largely to Russia to shifting pension-plan asset allocations favoring longer-dated U.S. government debt, have fueled a surprising drop in yields.

“The three main drivers of Treasury rates are economic growth, inflation and the base rate of interest; namely, the Federal Funds rate,” Anthony Parish, vice president of research and portfolio strategy for Sage Advisory, told ETF.com.

“The last of the three is expected to stay low for the foreseeable future,” said Parish. “Inflation expectations continue to be low. Meanwhile, economic conditions softened during the first quarter, driven by the cold winter. Low Treasury rates reflect this dynamic.”

While the Fed is somewhat puzzled by the lack of inflation, some see a “secular stagnation” related to ongoing debt reduction following the 2008 credit crisis, aging populations and technology-driven declines in available jobs. That said, there’s a general view that rates are slowly trending higher now, and that the slowness of the upward grind is to the advantage of longer-dated, higher-yielding debt.

Indeed, it’s mid- to long-end of the duration curve that’s attracting the most attention from yield-hungry investors. In the month of April, for example, the iShares 20+ Year Treasury Bond ETF (TLT | A-76) stood out, gathering a net of $426 million, making it the second-most-popular fixed-income ETF last month.

The $3.75 billion fund has now seen total net creations of $1.56 billion year-to-date. TLT, which has an effective duration of about 17 years, has also staged an impressive rally so far this year, climbing 11.5 percent.

Those gains stand in contrast to a far more modest rally in the SPDR S&P 500 ETF (SPY | A-97), which has gained slightly more than 2 percent year-to-date.